Ratchet-based ion pumping membrane systems

ABSTRACT

Described herein is an ion pump system implementing an electronic ratchet mechanism produced by modulating a spatially varying electric potential distribution that can result in a net ionic current and voltage. The ion pumping membrane system includes an ion-permeable layer that can also be integrated with ion-selective membranes. The electric potential distribution within the ion-permeable layer is modulated through external stimuli. When immersed in solution, ions within the ion-permeable layer experience a time varying, spatially asymmetric electric field distribution resulting in ratchet-driven direct ion pumping, which can be used in applications such as desalination.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part and claims benefit of U.S.patent application Ser. No. 16/907,076, filed Jun. 19, 2020 andPCT/US2020/038814, filed Jun. 19, 2020, both of which claim benefit toU.S. Provisional Application No. 62/863,761, filed Jun. 19, 2019, thespecification(s) of which is/are incorporated herein in their entiretyby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.DE-AC02-05CH11231 and DE-SC0004993 awarded by the Department of Energy.The government has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to ion pumps, namely, to devices thatimplement ratchet-based mechanisms to pump ions.

Background Art

Ion pumps are devices that use power to introduce a net ionic flux. Forexample, in cell membranes, nanoscale channels transduce the free energyavailable from adenosine triphosphate (ATP) hydrolysis to pump differenttypes of ions against a gradient in their electrochemical potential. Innatural photosynthetic processes, light is used to pump protons againsta gradient in their electrochemical potential so that the stored ionicenergy can be used by ATP synthase to produce energy sources required inthe Calvin Cycle to synthesize sugars. Although widely used in Nature,there are very few synthetic technologies that rely on ions for sensingand signaling, power generation, and energy storage and use.

Related Prior Demonstrations of Ionic Devices

Ion pumps are driven by a stimulus. While several stimuli are feasible,electricity and light are two common means. Electricity has been used invarious ion pump designs to drive electrodialysis, capacitivedeionization, ion intercalation deionization,^(1,2) and relatedapplications. In other device designs, spatially asymmetric electricpotential distributions are beneficial because they provide ease bywhich sensing and directional ion transport can occur where, inparticular, diodes that rectify ionic current serve as basic buildingblocks. Ionic diodes have been demonstrated by Bockris and co-workersusing bipolar membranes that contain a built-in asymmetric electricpotential distribution,³ and Rolandi, Gorodetsky, and colleagues usingbiological and biomimetic structures that were shown to serve as ionictransistors.^(4,5) Nanoporous materials have also been shown to exhibitionic current rectification in the presence of modest saltconcentrations due to the surface charge of the sub-10 nm pores impartedby the chemical functionalities that line the inner walls. Thesematerials are different from the other diodes in that there is little tono net electric potential drop across the membrane in the absence of apolarization bias. However, application of an external bias to induceionic current flow results in an enrichment or depletion of ions inregions of the sub-10 nm charged pores and ultimately ionic currentrectification. The most successful demonstrations by Siwy, Jiang, andcolleagues rely on conical nanopores etched in polymer films orSiN_(x).⁶⁻¹¹ While demonstrating both ionic diode rectification andionic transistor behavior, this group of ion pumps relies on externalbiases and chemical reactions to facilitate net ionic transport.

Photoelectrochemical-based ion pumps have also been demonstrated. Somenotably examples include the work of Ardo and co-workers usedphotoacid-dye-sensitized ion exchange membranes to demonstrate lightinduced proton pumping,¹²⁻¹⁵ Antonietti and co-workers fabricated alight actuated ion pump made of carbon nitride nanotube membranes,¹⁶ Gauand co-workers introduced light driven proton pumping through dopedJanus graphene oxide membranes,¹⁷ and Bakker and co-workers¹⁸illuminated different sides of a spiropyran doped membrane withdifferent wavelengths introducing different chemical transformations oneach side of the membrane that shuttled protons across it.

Related Prior Demonstrations of Electronic Flashing Ratchets

A ratchet is an asymmetric device whereby through use of an alternatingstimulus, net directional species transport results. Some well-knownexamples of ratchets include a car jack where pushing a lever up anddown results in an upward motion of the car, ATP synthase powered byproton-motive-force-driven ion translocation through a membrane in orderto drive formation of unfavorable ATP from ADP and P_(i), and arectifying electronic diode powered by an AC voltage to result in net DCcurrent. Each of these ratchets is considered a tilting ratchet, or arocking ratchet,^(19,20) because the force that results in net speciestransport is applied external to the entire pump, thus “tilting” theelectric potential distribution within it.

Electronic ratchets are devices that utilize modulations of spatiallyasymmetric electric potential distributions to drive a non-zero temporalmean current.²¹⁻²⁷ Electronic ratchets have been demonstrated boththeoretically and experimentally^(21,22,24-28) using induced chargeelectrokinetics to drive net transport of uncharged species,²⁹alternating electronic polarization to drive net electroniccurrent,^(21-28,30,31) and alternating redox reactions to drive netionic current.^(6,7) In flashing ratchets the force that results in netspecies transport is applied internal to the pump.^(22,32,33) To date,no theoretical or experimental reports exist that describe the presentinvention to use alternating electronic polarization to drive net ioniccurrent using a flashing ratchet mechanism, by what is referred toherein as an electric-polarization-induced ionic ratchet (EPIIR).

FIG. 3A shows a schematic illustration of the operating principles of aflashing ratchet.²³ Open circles mark the position of the chargedparticles at the beginning of every step and blue circles mark theposition of the charged particles at the end of every step. The sawtoothpotential distribution through the device (solid blue line) is switchedbetween two states such that in every step of the process potentialmaxima turn into potential minima, and vice versa.

In the initial step, t₁, the charged particles rest at the potentialminimum at x₁. Upon potential switching, the charged particles flowtoward the potential minima at x₀′ and x₁′. However, since the gradientin electric potential is steeper in one direction than the other, theparticles flowing toward x₁′ will reach this potential minimum beforethe particles flowing toward x₀′. When the potential is switched again,half of the particles at x₁′ will continue flowing to the right towardthe potential minimum at x₂. If the potential is switched before theparticles flowing to the left reach x₀′, most of these particles willreturn back and settle again in x₁. Since all charged particles areeither transported to x₂ or remain in x₁, there is a net particle fluxto the right. Here the present invention utilizes ratcheting principlesto demonstrate a first of its kind ratchet-based ion pump.

Applications of Ionic Processes

Controlling ion transport in solutions is critical to many societalchallenges, including generation of clean water for human consumptionand agriculture, recycling electronics and spent nuclear fuel forrecovery, concentration, and reuse of rare and expensive materials andchemicals, sensors, development of technologies that rely onmachine-biological interfaces, among others. Electricity-based devicesare timely due to the widespread increased electrification of societythrough installation of renewables to replace historical energy sourcesbased on fossil fuels, as well as their simplicity and lack of movingparts that are of increased interest for use in next-generationdistributed energy, water, and sensor technologies. Through thecombination of electricity and custom-engineered membranes, one canspecifically tune the selectivity and efficacy of a variety of ionicprocesses.

One non-limiting example application is for water desalination, whichrequires a process that generates a large enough free energy to drivethe unfavorable separation of water and/or salt from saline water. Theamount of free energy at room temperature to overcome a tenfold increasein monovalent ion activity, which is often approximated byconcentration, is equal to an electric potential of 0.118 V=(59 mV×2ions), and a modest potential of ˜0.21 V is sufficient to maintain a60-fold concentration gradient, which is approximately the saltconcentration ratio between sea water and potable water. Commercialelectrodialysis achieves ion pumping and desalination by driving redoxreactions, which waste voltage as kinetic overpotentials for the redoxreactions and require redox-active species in the saline water.Capacitive deionization achieves ion pumping and desalination throughinterfacial charging, but requires that desalination stops when thecharge capacity of the electrodes is reached, thus requiring that thewater flow streams be switched from saline to desalted in order todischarge the salt and enable further operation.

As another non-limiting example application, membranes can be used forion separation whereby an external force acts on species to drive themthrough the membrane with chemical specificity based on total charge,size, and/or local charge density. This force can arise from an appliedpressure, gravity, temperature gradients, or others, but of particularincreased interest are electric fields because of their simplicity inbeing generated at electrified interfaces using electricity or light.Electricity-driven separations are by nature electrochemical andtherefore are driven by charging interfaces between electrically andionically conductive media. They can be implemented as either batchprocesses, where interfacial (pseudo)capacitive charging reactions aresubsequently coupled to interfacial (pseudo)capacitive dischargingreactions, or continuous processes, where sustained currents are enabledthrough direct coupling of interfacial charging reactions withheterogeneous redox reactions between electrodes and liquidelectrolytes. Non-electrical separations processes have drawbacks inthat typically they require large amounts of solvents to provide themedium for classical separations or the addition of chemical species,such as chelates or sorbents, to the water in order to change theproperties of dissolved species and alter their solubility in a phase.

Continuous electrochemical processes are simple to engineer because theyrequire no moving parts or mechanical switches, yet when driven by redoxreactions, they require the presence of soluble chemical species toserve as reactants and products of each redox reaction. Requiringadditional soluble chemical species complicates this otherwisesimplistic design and often times is a nonstarter for separationstechnologies because the added chemical species may not be compatiblewith desired processes or ultimate uses, e.g. adding molecules todrinking water. While batch electrochemical processes require morecomplex maintenance of flow streams due to the limited capacity of(pseudo)capacitive charging/discharging reactions, the simplicity ofthese interfacial reactions and lack of additional soluble chemicalspecies are desirable traits.

Besides water desalination and separations, a ratchet-based ion pump canbe used in many other applications. Some non-limiting examples includefine control of ion flow out of a reservoir in a drug delivery systemand to increase the sensitivity of chemical sensors. Moreover, there issignificant recent interest in trying to mimic logic processes inneurons and the brain via neuromorphic computing, and developmachine-biology interfaces. In order to better mimic Nature's braincircuitry, several groups are working to design, demonstrate, andcontrol ionic transistors.^(8,10,34-39)

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to provide systems anddevices that make use of a ratchet mechanism to pump ions in steadystate without necessarily introducing electrochemical reactions, asspecified in the independent claims. Embodiments of the invention aregiven in the dependent claims. Embodiments of the present invention canbe freely combined with each other if they are not mutually exclusive.

In some aspects, the present invention features an ion pump based on anelectronic ratchet mechanism. By removing the requirement to drive redoxreactions, which is required in commercial electrodialysis, the presentinvention does not waste any voltage to undesired redox reactions, doesnot corrode due to redox, and does not generate redox products thatcould affect the water sources. By allowing for continuous operation,the system does not need to stop desalinating during operation, unlikein capacitive deionization which must switch the water flow streams fromsaline to desalted after the ionic capacitor or battery is fully chargedor discharged.

Electronic ratchets are devices that utilize modulation produced byspatially varying electric fields to drive a non-zero temporal averagecurrent. Similar to peristaltic pumps, where the pump mechanism is notin direct contact with the pumped fluid, electronic ratchets induce netcurrent with no direct charge transfer between the power source and thepumped charge carriers. Thus, electronic ratchets can be used to pumpions in steady state with no electrochemical reactions between the powersource and the pumped ions, resulting in an ion pump that is more energyefficient than any other electrochemical deionization/desalinationdesign.

As will be described herein, electric-polarization-induced ionic ratchet(EPIIR)-based ion pumps were numerically simulated. Computations showthat the ratchet mechanism facilitates net ion pumping and that themagnitude of the ionic current is heavily dependent on the frequency andshape of the input signal. The optimal frequency for current generationis directly related to ion diffusivity and inversely related to the timeconstant for charging and discharging, but also depends on otherparameters such as the shape of the electric potential distribution, thewidth of the electric potential distribution, among others.

EPIIR-based ion pumps were fabricated by coating the two outer surfacesof nanoporous alumina wafers with gold forming a nano-porous capacitor.Electric fields within the nanopores were altered by modulating thevoltage applied across the capacitor. When immersed in solution, ionswithin the pores experienced time varying electric fields resulting inratchet-based ion pumping. The device pumping performance was evaluatedfor various input signals, geometries and solutions. The proposeddevices may be used as building blocks in a wide range of applicationssuch as for water desalination, concentration of salts, chemicalseparations, sensors, artificial photosynthesis, and many others. Thisarchitecture provides a straightforward path for devices with a verylarge active area that would be difficult to realize with other recentratchet demonstrations.

According to some embodiments, the present invention features an iontransport structure that comprises an ion-permeable layer coupled to atleast two contacts. The ion transport structure is configured totransport ions across an ion-permeable layer when a spatially asymmetricelectric potential distribution is temporally modulated to changeelectric fields within the ion transport structure, resulting in aratchet-driven ion pump. In some embodiments, the spatially asymmetricelectric potential distribution is temporally modulated by theapplication of a stimulus or perturbation to the structure to alter theelectric fields. The stimulus or perturbation may be an electrical bias,light, a temperature gradient, or a pH gradient. The ion transportstructure may be configured to continuously transport ions usingalternating electronic polarization. Without wishing to limit thepresent invention, the ion transport structure can pump ions withminimized resistance and without using electrochemical reactions ormechanical forces.

In some embodiments, the ion-permeable layer comprises a dielectricmaterial, a semiconductor, a polymer, or an ion-selective material. Insome embodiments, the contact is a layer or wire comprised of anelectrically conductive material. In some embodiments, a plurality ofchannels is disposed through the ion transport structure.

According to another embodiment, the present invention features a methodof moving ions. The ions are moved in a solution by providing an iontransport structure as described herein, placing the ion transportstructure in the solution, and temporally modulating a spatiallyasymmetric electric potential distribution to change electric fieldswithin the ion transport structure to transport ions across theion-permeable layer.

In other embodiments, the present invention features an ion transportstructure comprising a plurality of ion-permeable layers and a pluralityof contacts that form a stack in which the ion-permeable layersalternate with the plurality of contacts. The ion transport structure isconfigured to transport ions through the stack when a spatiallyasymmetric electric potential distribution is temporally modulated tochange electric fields within the ion transport structure, resulting ina ratchet-driven ion pump. The ion transport structure may include aplurality of channels disposed through the stack of alternating layers.

In some embodiments, the present invention comprises an ion pumpingsystem that comprises an ion transport structure. The ion transportstructure comprises an ion-permeable layer and at least two contactscoupled to the ion-permeable layer, and a first and a secondion-selective membrane that are operatively coupled to the ion transportstructure. The first ion-selective membrane and the second ion-selectivemembrane are each selective for ions having a specific charge. The ionpumping system may also include a plurality of channels disposed throughthe ion transport structure.

Without wishing to be bound to a particular theory or mechanism, theions in solution are transported across the ion pumping system when aspatially asymmetric electric potential distribution is temporallymodulated to change electric fields within the ion transport structure,resulting in a ratchet-driven ion pump. In some embodiments, thespatially asymmetric electric potential distribution is temporallymodulated by application of a stimulus or perturbation to the structureto alter the electric fields within the ion pumping system. The stimulusor perturbation may be an electrical bias, light, a temperaturegradient, or a pH gradient. The ion transport structure is configured tocontinuously transport ions using alternating electronic polarizationwith minimized resistance and without using electrochemical reactions ormechanical forces.

In one embodiment, the first ion-selective membrane is disposed on theion transport structure and the second ion-selective membrane isattached to the ion transport structure such that the secondion-selective membrane and ion transport structure are side by side. Inanother embodiment, the first ion-selective membrane and secondion-selective membrane are disposed side by side on the ion transportstructure.

In some other embodiments, the at least two contacts in the ion pumpingsystem comprise two sets of interlaced contact fingers, and each set isconnected to different channels. A first set of strips comprising thefirst ion-selective membrane are disposed on one set of contact fingersand a second set of strips comprising the second ion-selective membraneis disposed on the other set of contact fingers such that the strips ofthe first ion-selective membrane alternate with the strips of the secondion-selective membrane. Each set of contact fingers has a correspondingset of interlaced contact fingers disposed on the second surface of theion-permeable substrate and are connected to the same channels, therebyforming a paired set. Each paired set is connected to its own separatepower source.

In some embodiments, the present invention comprises a deionizationsystem for moving ions in a solution from a first compartment to asecond compartment. The deionization system may comprise an ion pumpingsystem described herein. The ion pumping system separates the first andsecond compartment. Each compartment may contain the solution having aninitial concentration of ions. When a spatially asymmetric electricpotential distribution is temporally modulated to change electric fieldswithin the ion transport structure, resulting in a ratchet mechanism,ions from the first compartment are selectively transported in onedirection across the ion pumping system, thereby increasing the ionconcentration in the second compartment and reducing the ionconcentration in the first compartment.

In one embodiment, the desalination system may comprise the first ionselective membrane, which is disposed on the ion transport structure,and the second ion-selective membrane, which is attached to the iontransport structure, to form a single, continuous barrier that separatesthe first and second compartment. In another embodiment, the first ionselective membrane is disposed on the ion transport structure, and thesecond ion-selective membrane is disconnected from the ion transportstructure. Each ion-selective membrane forms a barrier that separatesthe first and second compartment.

One of the unique and inventive technical features of the presentinvention is the continuous separation of ions using alternatingelectronic polarization to drive net ionic current. Without wishing tolimit the invention to any theory or mechanism, it is believed that thistechnical feature advantageously provides for the continuous separationof ions by using electricity and without requiring additional processes.None of the presently known prior references or work has the uniqueinventive technical feature of the present invention. Furthermore, theprior references teach away from the present invention. The currentbelief is that a redox reaction or change in flow streams is needed tohave a continuous flow of ions. Existing processes that use electricityto drive ion transport and separation require the performance of anadditional process to drive net ionic current, such as redox reactions,as are required in electrodialysis, and changing flow streams, as isrequired in capacitive deionization. Contrary to current teachings,surprisingly the inventors can drive net ionic current by onlymodulating electronic current into and out of the device. The presentinvention can pump ions with no redox reactions and without changingflow streams.

Any feature or combination of features described herein are includedwithin the scope of the present invention provided that the featuresincluded in any such combination are not mutually inconsistent as willbe apparent from the context, this specification, and the knowledge ofone of ordinary skill in the art. Additional advantages and aspects ofthe present invention are apparent in the following detailed descriptionand claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The patent application or application file contains at least one drawingexecuted in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

The features and advantages of the present invention will becomeapparent from a consideration of the following detailed descriptionpresented in connection with the accompanying drawings in which:

FIG. 1A is a cross-sectional view of a non-limiting embodiment of anEPIIR-based ion pumping system comprised of an ion transport layer withat least one conductive layer on its surface. Ions are separated byapplying an alternating voltage between the conductive layer and thesolution on the other side of the ion transport layer. In someembodiments, a wire contact can be immersed in the solution.

FIG. 1B is another non-limiting embodiment of an EPIIR-based ion pumpingsystem comprised of an ion transport layer with at least one conductivelayer on each of its surfaces. Ions are separated by applying analternating voltage between the conductive layers.

FIG. 1C shows a schematic illustration of an EPIIR-based ion pumpingsystem comprised of stacked ion transport layers with conductive layersbetween every two transport layers and on the two surfaces. Ions areseparated by applying an alternating voltage between the conductivelayers. In a non-limiting embodiment, connections to the layers canalternate between positive and negative, although it is not required. Itis to be understood that in other embodiments, the connections can varydepending on the geometry.

FIG. 1D shows a schematic illustration of an EPIIR-based ion pumpingsystem comprised of an ion transport layer with a conductive layer oneach of its surfaces. An anion selective exchange layer is deposited ontop of the EPIIR resulting in selective ion pumping. A cation exchangelayer is placed near the EPIIR to allow the transport of cationsresulting in a directed transport of both ion types from one side of thesystem to the other.

FIG. 1E shows a schematic illustration of an EPIIR-based ion pumpingsystem comprised of an ion transport layer with at least one conductivelayer on its surface. A cation selective exchange layer is deposited ontop of the EPIIR ion pump resulting in selective ion pumping. An anionexchange layer is placed near the EPIIR to allow the transport of anionsresulting in a directed transport of both ion types from one side of thesystem to the other.

FIG. 1F shows a schematic illustration of an EPIIR-based ion pumpingsystem comprised of two ion transport layers with separate conductivelayers on each of their surfaces. A cation selective exchange layer isdeposited on top of one EPIIR and an anion selective exchange layer isdeposited on top of the second EPIIR thus each of the EPIIRpredominantly pumps only one type of ion. Desalination is achieved wheneach of the EPIIRs is operated with a different voltage source.

FIG. 1G shows a schematic illustration of an EPIIR-based ion pumpingsystem comprised of an ion transport layer with at least one conductivelayer on each of its surfaces. Different-signed ions are pumped in thesame direction by applying an optimized input signal frequency based onthe diffusivity of the ions.

FIG. 1H shows a schematic illustration of an EPIIR-based ion pumpingsystem comprised of an ion transport layer with at least one conductivelayer on each of its surfaces. Ion modulation or transport in thestructure induces water modulation and transport, resulting in anelectro-osmotic water pump that is most effective at a specificoptimized input signal frequency.

FIGS. 2A-2B are schematics of the region at the vicinity of a pore in anEPIIR ion pump according to two embodiments of the present inventionwith the alternating electric potential, V, occurring at both contactsin FIG. 2A and at one contact in FIG. 2B. Ionic voltage, V_(c), can bemeasured using stable reference electrodes, such as Ag/AgCl or saturatedcalomel electrode.

FIG. 3A shows the operating principles of a flashing ratchet.²³ Thesolid blue line illustrates the spatial electric potential distribution(x_(i)) during three sequential time steps (t_(i)). Modulation of thespatial electric potential distribution occurs from an initial state,t₁, to t₂ where then open circles mark the initial position of thecharged particles at t₂ and filled circles mark the position of thecharged particles after some time has passed. At ta, the spatialelectric potential distribution is returned to its initial distributionand charged particles initially at positions indicated by open circlestransport to positions indicated by closed circles, illustrating nettransport to the right corresponding to net current on going from t₁ tot₃.

FIG. 3B shows the operating principles of an ion permeable EPIIR at onecontact. This scheme is similar to that shown in FIG. 3A, although thespatial electric potential distribution is shown in magenta. When thevoltage is switched from V⁻ to V⁺ ions disperse in both directionsalbeit at different rates. If the voltage is switched back to V beforeall ions have reached their steady state distribution, some of theleft-flowing ions will be pulled into the potential well that is formednear the contact. Many right-flowing ions will not be pulled back intothe potential well. The right-flowing ions transport faster due to thesharper gradient in potential and thus will be further away from thecontact and generate net transport to the right corresponding to netcurrent flow.

FIG. 4A shows the theoretical calculated spatial electric potentialdistribution near the EPIIR high voltage contact at various distancesfrom the pore center, r, for an applied voltage of 0.4 V for thesimulation domain shown in FIG. 2A.

FIG. 4B shows the weighted average spatial electric potentialdistribution near the high voltage contact calculated using thepotential distributions shown in FIG. 4A. The inset shows the potentialfurther away from the contact.

FIG. 4C shows the calculated EPIR net ionic current, assumingshort-circuit conditions for the transported ions, as a function of theinput signal frequency and duty cycle. The potential distribution is asshown in FIG. 4B.

FIG. 5A shows the calculated EPIIR net ionic current, assumingshort-circuit conditions for the transported ions, as a function of theion diffusivity for several input signal frequencies.

FIG. 5B shows the potential distribution used to calculate the datapresented in FIG. 5A.

FIG. 6A shows a scanning electron micrograph plan view of an EPIRfabricated from an anodized aluminum oxide (AAO) substrate with goldcontacts.

FIG. 6B is a non-limiting schematic of a measurement setup to detectionic voltage, V_(out), based on an input ratchet signal, V_(c). Ioniccurrents can be measured by replacing the voltmeter, V, with alow-impedance ammeter.

FIGS. 6C-6E show a non-limiting design of a test cell used to pump ionsthat uses an EPIR to generate an ionic voltage that is then used toforce directional transport of anions and result in net deionizationwhen the solution level is raised as shown in FIG. 6E, left.

FIG. 7A shows the ionic open-circuit voltage measured between twoAg/AgCl wires across an EPIIR with nominally 40 nm diameter pores withinput signals at various duty cycles. The aqueous electrolyte solutionis 1 mM KCl and the input signal is a square wave switched between −0.2V and 0.2 V at a frequency of 100 Hz.

FIG. 7B shows the ionic open-circuit voltage measured between twoAg/AgCl wires across an EPIIR with nominally 40 nm diameter pores as afunction of the input signal amplitude (peak to peak) for aqueouselectrolyte solutions of 1 mM KCl and 10 mM KCl. The input signal is asquare wave with a duty cycle of 50%, and a frequency of 100 Hz.

FIGS. 7C-7D show the ionic open-circuit voltage measured between twoAg/AgCl wires across an EPIIR with nominally 40 nm diameter pores as afunction of the input signal frequency and duty cycle. The input signalis a square wave switched between −0.2 V and 0.2 V and the aqueouselectrolyte solution is 1 mM KCl (FIG. 7C) and 10 mM KCl (FIG. 7D).

FIG. 8A shows the ionic open-circuit voltage measured between twoAg/AgCl wires across an EPIIR with nominally 40 nm diameter pores forfrequencies of 1 Hz, 10 Hz, and 100 Hz. The aqueous electrolyte solutionis 10 mM HCl and the input signal is a square wave switched between 0 Vand 0.8 V with a duty cycle of 0.5.

FIG. 8B shows the measured ionic short-circuit current density betweentwo Ag/AgCl wires across the EPIR with the same conditions as in FIG.8A.

FIGS. 8C-8D show the ionic open-circuit voltage measured between twoAg/AgCl wires for a single period (FIG. 8C) and the sum of eachhalf-period (FIG. 8D) across the EPIIR as a function of time with thesame conditions as in FIG. 8A.

FIGS. 8E-8F show the measured ionic short-circuit current densitybetween two Ag/AgCl wires for a single period (FIG. 8E) and the sum ofeach half-period (FIG. 8F) across the EPIIR as a function of time withthe same conditions as in FIG. 8A.

FIGS. 9A-9E show schematic illustrations of a pore in an EPIIR ion pumpfor a single device (FIG. 9A), stacked EPIIR (FIG. 9B) and the spatialelectric potential distribution within the pore (FIG. 9C), and stackedEPIIR operated as a reversible ratchet (FIG. 9D) and the spatialelectric potential distribution within the pore (FIG. 9E).

FIG. 10 is a non-limiting schematic of the region at the vicinity of apore in an EPIIR ion pump made of silicon with a p-i-n dopingconfiguration for light-induced modulation of the electric potential,V_(Si), across the pore.

FIGS. 11A-11B show a top view (FIG. 11A) and a 3D representation (FIG.11B) of non-limiting embodiments of an interdigitated EPIIR ion pump forminimized resistance, two compartment desalination.

FIG. 12 shows Na⁺ and Cl⁻ short-circuit current densities as a functionof the input signal frequency. The input signal is g(t)=0.5(1+sin(2πft)) and the relative position of the contact is at 0.99 of thespatial period.

FIG. 13 shows the single-membrane optimized ambipolar current density asa function of the relative position of the contact in the spatialperiod, and the input frequency that corresponds to each condition.

DETAILED DESCRIPTION OF THE INVENTION

Following is a list of elements corresponding to a particular elementreferred to herein:

-   -   100 ion pumping system    -   110 ion transport structure    -   112 ion-permeable layer    -   114 first surface of the ion-permeable layer    -   115 channels    -   116 second surface of the ion-permeable layer    -   122 first contact    -   124 second contact    -   130 first ion-selective membrane    -   135 second ion-selective membrane    -   140, 145 power source

As used herein, the term “contact” refers to an electrically conductivematerial. In some embodiments, the electrically conductive contact maybe in the form of a contact layer or a wire in solution. There may be aplurality of contacts coupled to the ion transport structure, but thereshould be at least one positive contact and one negative contact.

As used herein, the term “asymmetric electric potential distribution”refers to a spatial variation in electric potential in which thesepotential variations in the two halves of the structure between thecontacts are not mirror images of each other.

As used herein, the term “diffusivity” is related to the rate at whichions or molecules spread through a phase. Diffusivity is defined as ameasure of the mean squared displacement of a species over time. Theterm diffusivity applies to any mobile species in a phase, including butnot limited to, electrons in a semiconductor, O₂ in air, and ions in asolution.

Referring now to FIG. 1A, in some embodiments, the present inventionfeatures an ion transport structure (110) comprising an ion-permeablelayer (112) coupled to at least two contacts (122). Without wishing tolimit the present invention to a particular theory or mechanism, the iontransport structure (110) is configured to transport ions across theion-permeable layer (112) when a spatially asymmetric electric potentialdistribution is temporally modulated to change electric fields withinthe ion transport structure, resulting in ratchet-driven ion pumping.

As a non-limiting example, the ion transport structure (110) cantransport ions across the ion-permeable layer (112) when a stimulus orperturbation, such as an electrical bias, light, a temperature gradient,or a pH gradient, is applied to the structure. Moreover, the iontransport structure (110) can pump salt with minimized resistance andwithout electrochemical reactions or mechanical forces, such as pressureor stress.

Referring now to FIG. 1G, in one embodiment the present inventionfeatures an ion transport structure (110) comprising an ion-permeablelayer (112) coupled to at least two contacts (122). Without wishing tolimit the present invention to a particular theory or mechanism, the iontransport structure (110) is configured to transport different-signedions in the same direction across the ion-permeable layer (112) when aninput signal frequency is applied to change electric fields within theion transport structure, resulting in a ratchet-driven ion pump. In someembodiments, the input signal frequency can range from about 1 GHz-1MHz, 1 MHz-1 kHz, 1 kHz-1 Hz, or 1 Hz-1 mHz.

In other embodiments, the ion transport structure (110) is configured tocontinuously transport ions using alternating electronic polarization.In this configuration, the ion transport structure (110) pumps ions withminimized resistance and without using electrochemical reactions ormechanical forces. In further embodiments, the ion-permeable layercomprises a dielectric material, a semiconductor, a polymer, or anion-selective material. In some embodiments, the contact (122) is alayer or wire comprised of an electrically conductive material. In otherembodiments, a plurality of channels (115) is disposed through the iontransport structure (110).

In some embodiments, the present invention features a method ofselectively moving one or more types of ions in a solution. In preferredembodiments, the method comprises providing the ion transport structure(110) described herein, placing the ion transport structure (110) in thesolution, and choosing an input signal frequency based on a diffusivityof the one or more types of ions in the solution. Without wishing tolimit the present invention to a particular theory or mechanism, theinput signal frequency modulates a spatially asymmetric electricpotential distribution to change electric fields within the iontransport structure to optimally transport the one or more types of ionsacross the ion-permeable layer (112) in either direction. In someembodiments, the input signal frequency can range from about 1 GHz-1MHz, 1 MHz-1 kHz, 1 kHz-1 Hz, or 1 Hz-1 mHz.

In further embodiments, the one or more types of ions in the solutionare of a same charge type and are transported in opposite directions,resulting in steady state ion pumping. In other embodiments, the one ormore types of ions in the solution are of opposite charge types and aretransported in the same direction, resulting in steady state ionpumping. As a non-limiting example, if an aqueous salt solution iscomposed of anions and cations with different diffusivities, forexample, Na⁺ and Cl⁻ with diffusivities of 1.33×10⁻⁵ cm²/s and1.998×10⁻⁵ cm²/s,^(44,45) respectively, there will be a frequency rangeover which current reversal will cause cations and anions to travel inthe same direction. Other non-limiting examples of anions and cationswith different diffusivities include K⁺ and I⁻ with diffusivities of1.960×10⁻⁵ cm²/s and 2.045×10⁻⁵ cm²/s, respectively. In someembodiments, the transport of ions in the solution causes a net flux ofwater, thereby pumping water across the ion transport structure.

In some embodiments, the present invention features an ion transportstructure (110) comprising a plurality of ion-permeable layers (112) anda plurality of contacts (122) forming a stack in which the ion-permeablelayers (112) alternate with the plurality of contacts (122). In thisconfiguration, the ion transport structure is configured to transportions through the stack when an input signal frequency is applied tochange electric fields within the ion transport structure, resulting ina ratchet-driven ion pump. In further embodiments, a plurality ofchannels is disposed through the stack of alternating layers. In furtherembodiments, the input signal frequency can range from about 1 GHz-1MHz, 1 MHz-1 kHz, 1 kHz-1 Hz, or 1 Hz-1 mHz.

In further embodiments, the present invention features an ion pumpingsystem (100). The ion pumping system comprises an ion transportstructure (110), a first ion-selective membrane (130), and a secondion-selective membrane (135). In some embodiments, the ion transportstructure (110) comprises an ion-permeable layer (112) and at least twocontacts (122) coupled to the ion-permeable layer (112). In otherembodiments, the first ion-selective membrane (130) is operativelycoupled to the ion transport structure (110), and the secondion-selective membrane (135) is operatively coupled to the ion transportstructure (110). In further embodiments, the first ion-selectivemembrane (130) and the second ion-selective membrane (135) are eachselective for ions having a specific charge, and the ions aretransported across the ion transport structure (110) and the secondion-selective membrane (135) when an input signal frequency is appliedto change electric fields within the ion transport structure, resultingin a ratchet-driven ion pump. In some embodiments, the input signalfrequency can range from about 1 GHz-1 MHz, 1 MHz-1 kHz, 1 kHz-1 Hz, or1 Hz-1 mHz.

In some embodiments, the ion transport structure is configured tocontinuously transport ions using alternating electronic polarization.In this configuration the ion transport structure (110) pumps ions withminimized resistance and without using electrochemical reactions ormechanical forces.

In other embodiments, the first ion-selective membrane is disposed onthe ion transport structure and the second ion-selective membrane (135)is attached to the ion transport structure (110) such that the secondion-selective membrane (135) and ion transport structure (110) are sideby side. In some embodiments, a plurality of channels (115) is disposedthrough the ion transport structure (110). In further embodiments, theat least two contacts (122) comprise two sets of interlaced contactfingers, each set connected to different channels. A first set of stripscomprising the first ion-selective membrane (130) are disposed on oneset of contact fingers and a second set of strips comprising the secondion-selective membrane (135) is dispose on the other set of contactfingers such that the strips of the first ion-selective membrane (130)alternate with the strips of the second ion-selective membrane (135).Each set of contact fingers has a corresponding set of interlacedcontact fingers disposed on the second surface (116) of theion-permeable substrate and connected to the same channels, therebyforming a paired set, and each paired set is connected to its ownseparate power source.

In other embodiments, the present invention features a deionizationsystem for moving ions in a solution from a first compartment to asecond compartment. The deionization system comprises the ion pumpingsystem (100) described herein. In this configuration the ion pumpingsystem separates the first and the second compartments. Each compartmentcontains the solution having an initial concentration of ions and aninput signal frequency is applied to change electric fields within theion transport structure, resulting in a ratchet mechanism. Ions from thefirst compartment are selectively transported in one direction acrossthe ion pumping system, thereby increasing the ion concentration in thesecond compartment and reducing the ion concentration in the firstcompartment. In some embodiments, the input signal frequency can rangefrom about 1 GHz-1 MHz, 1 MHz-1 kHz, 1 kHz-1 Hz, or 1 Hz-1 mHz.

In further embodiments, the deionization system is configured such thatthe first ion selective membrane (130) is disposed on the ion transportstructure (110) and the second ion-selective membrane (135) is attachedto the ion transport structure (110) to form a single, continuousbarrier that separates the first and second compartment. In otherembodiments, the first ion selective membrane (130) is disposed on theion transport structure (110), wherein the second ion-selective membrane(135) and the ion transport structure (110) are disconnected, and eachone forms a barrier that separates the first and second compartment.

Referring to FIG. 1C, in other embodiments, the present inventionfeatures an ion transport structure (110) comprising a plurality ofion-permeable layers (112) and a plurality of contact layers (122)forming a stack in which the ion-permeable layers (112) alternate withthe plurality of contacts (122). The ion transport structure (110) isconfigured to transport ions through the stack when a spatiallyasymmetric electric potential distribution is temporally modulated tochange electric fields within the ion transport structure, resulting inratchet-driven ion pumping, preferably with minimized resistance andwithout electrochemical reactions or mechanical forces, such as pressureor stress. In other embodiments, the ion transport structure (110) isconfigured to transport different-signed ions in the same directionthrough the stack when a spatially asymmetric electric potentialdistribution is temporally modulated at a specific frequency to changeelectric fields within the ion transport structure, resulting in aratchet-driven ion pump.

In other embodiments, the present invention features a method forselectively moving one or more ions in a solution using the iontransport structure (110) described herein. The method comprisesproviding the ion transport structure, placing the ion transportstructure in the solution, and applying an input frequency that waschosen based on the diffusivity of one or more ions in the solutionthereby temporally modulating a spatially asymmetric electric potentialdistribution to change electric fields within the ion transportstructure to transport ions across the ion-permeable layer. Referringnow to FIG. 1H, in some embodiments, the present invention features amethod for pumping water. Without wishing to limit the present inventionto any theory or mechanism, the ion modulation or transport in thestructure induces water modulation and transport, resulting in anelectro-osmotic water pump that is most effective at a specificoptimized input signal frequency.

In some embodiments, the ion-permeable layer may comprise a dielectricmaterial, a semiconductor, a polymer, or an ion-selective material. Onenon-limiting example of the dielectric material is alumina. Anon-limiting example of the semiconductor is a silicon p-i-n junction.For the stacked ion transport structure (110), the plurality ofion-permeable layers may comprise a dielectric material, asemiconductor, a polymer, an ion-selective material, or combinationsthereof such that the layers are not necessarily the same material. Insome embodiments, the ion-permeable layers (112) can have the samethickness or vary in thickness.

In some embodiments, the electrically conductive contacts may comprisean electrically conductive material. The electrically conductivematerial may include, but is not limited to, a metal, conductivepolymer, highly doped semiconductor, among others.

In some embodiments, at least one contact is connected to the iontransport structure (110). In other embodiments, two or more contactsare connected to the ion transport structure (110).

In one embodiment, a plurality of channels (115) may be disposed throughthe ion transport structure (110). For example, the channels (115) canbe disposed through the stack of alternating layers. In conjunction withthe various embodiments, the channels (115) can span from one surface toan opposing surface. In one embodiment, the channels (115) may bestraight channels or pores. However, the channels do not necessarilyhave to be straight. In another embodiment, the channels may instead bea network of channels interconnected together to form a “sponge like”geometry.

In some embodiments, the channels (115) can have a diameter ranging fromabout 5 nm to about 500 nm. In other embodiments, the ion-permeablelayer (112) can have a porosity ranging from about 10% to about 50%,e.g. fraction of the void (i.e. “empty”) space or volume relative to thetotal volume of a material.

Referring to FIGS. 1D-1F, in some embodiments, the present inventionfeatures an ion pumping system (100) comprising the ion transportstructure (110) having a first ion-selective membrane (130) disposedthereon and a second ion-selective membrane (135). In furtherembodiments, a power source (140) may be operatively coupled to thefirst contact layer (122) and second contact layer (124). In anon-limiting embodiment, as shown in FIG. 1D, the power source (140) canbe connected to contact layers on both sides of the ion-permeable layer.In an alternative non-limiting embodiment, as shown in FIG. 1E, thepower source (140) can be connected to the contact layer on one sideonly, e.g. one lead is connected to the one of the contact layers andthe other lead is disposed in solution.

In one embodiment, the ion transport structure (110) may be according tothe embodiment in FIG. 1D, which includes an ion-permeable layer (112)having a first surface (114) and an opposing second surface (116), afirst contact layer (122) disposed on the first surface (114), a secondcontact layer (124) disposed on the second surface (116), and the firstion-selective membrane (130) disposed on the first contact layer (122).Alternatively, the first ion-selective membrane (130) is disposed on theouter contact layer of the stack of FIG. 1C.

In some embodiments, the power source (140) is configured to apply analternating electrical bias between the first contact layer (122) andthe second contact layer (124), which causes ions to be transportedacross the ion transport structure (110), which results inratchet-driven ion pumping, whose voltage can be used to drive ions ofopposite charge through the second ion-selective membrane (135). Withoutwishing to limit the present invention, the ion pumping occurs withminimized resistance and without electrochemical reactions. In otherembodiments, the system (100) may further comprise a second power source(145) operatively coupled to contact layers that are not coupled to thefirst power source (140). The second power source (145) can apply analternating electrical bias to said contacts.

In some embodiments, the first ion-selective membrane (130) and thesecond ion-selective membrane (135) are each selective for ions having aspecific charge. For example, the first ion-selective membrane (130) isa cation-exchange membrane and the second ion-selective membrane (135)is an anion-exchange membrane. Alternatively, the first ion-selectivemembrane (130) is an anion-exchange membrane and the secondion-selective membrane (135) is a cation-exchange membrane. In otherembodiments, ion-selective membranes (130) are made by asymmetricsurface modification of the ion-permeable layer (112) using couplingchemistries to bond chemical moieties found in ion-exchange membranesdirectly to the ion-permeable layer (112). For example, the chemicalmoieties may include, but are not limited to, functionalizedcarboxylates, phosphonates, borates, amines, imidazoliums, or aromaticgroups including alcohols.

In one embodiment, a plurality of channels (115) is disposed through theion transport structure (110). The plurality of channels (115) may bestraight channels or an interconnected network of channels. In someembodiments, the channels (115) can have a diameter ranging from about 5nm to about 500 nm. In other embodiments, the ion-permeable layer (112)can have a porosity ranging from about 10% to about 50%,

In one embodiment, as shown in FIGS. 1C-D, the second ion-selectivemembrane (135) may be attached to the ion transport structure (110) suchthat the second ion-selective membrane (135) and ion transport structure(110) are side by side. In another embodiment, the second ion-selectivemembrane (135) may be separate from the ion transport structure (110).

In yet another embodiment, as shown in FIG. 11A, the first contact layermay comprise two sets of interlaced contact fingers (122 and 124), eachset connected to different channels with no direct electrical contactbetween the two sets. A first set of strips comprising the firstion-selective membrane (130) may be disposed on one set of contactfingers and a second set of strips comprising the second ion-selectivemembrane (135) may be dispose on the other set of contact fingers suchthat the strips of the first ion-selective membrane (130) alternate withthe strips of the second ion-selective membrane (135). In furtherembodiments, each set of contact fingers has a corresponding set ofinterlaced contact fingers disposed on the second surface (116) of theporous substrate and connected to the same channels, thereby forming apaired set. Preferably, each paired set is connected to its own separatepower source.

In some embodiments, as shown in FIG. 1F, the first power source (140)is connected to a paired set of contact fingers, and the second powersource is connected to a second paired set of contact fingers indicatedby the dotted pattern. In some embodiments, each power source applies aspecific alternating electronic polarization. In other embodiments, thetwo power sources electronically interact to apply a series of two ormore alternating electronic polarizations between any combinations ofthe four interlaced contact fingers.

Various factors can affect the choice of voltage, for example, if thepores are different sizes or depending on the thickness of theion-permeable layer. In one embodiment, the same voltage source may beused for two sets of contacts with the contacts given oppositepolarities. In another embodiment, V1 can equal V2 if the duty cycle isdifferent or depending on the materials properties.

According to some embodiments, the ion-permeable layer may comprise adielectric material, a semiconductor or a polymer. One example of thedielectric material is alumina. A non-limiting example of thesemiconductor is a silicon p-i-n junction. For the stacked ion transportstructure (110), the plurality of ion-permeable layers may comprise adielectric material, a semiconductor, a polymer, or combinations thereofsuch that the ion-permeable layers are not necessarily the samematerial.

In other embodiments, the electrically conductive contacts may comprisean electrically conductive material. The electrically conductivematerial may include, but is not limited to, a metal, conductivepolymer, highly doped semiconductor, among others. For example, themetallic material may include, but is not limited to, gold, silver,copper, or metal alloys. As another example, the conductive polymermaterial may include, but is not limited to,poly(3,4-ethylenedioxythiophene), polyacetylene, or a composite materialsuch as a graphene-polymer mixture. As another example, the highly dopedsemiconductor material may include, but is not limited to, Si, II-Vs,II-IVs, perovskites, or organic semiconductors such aspoly(3-hexylthiophene). In one embodiment, the contacts may both be thesame electrically conductive material or may be different electricallyconductive materials. Because redox reactions are not intended to beperformed at the electrically conductive contacts, after theirdeposition the electrically conductive contacts, and even theion-permeable layer, can be coated in a chemically robust andelectrically insulating coating to extend the lifetime of the device. Asan example, the coating may include, but is not limited to, metal oxidessuch as alumina, silica, titania, and hafnia, and can be deposited byphysical vapor deposition, by atomic layer deposition,electrochemically, among other methods.

According to other embodiments, the present invention may feature adeionization system for moving two types of oppositely charged ions in asolution from a first compartment to a second compartment. Thedeionization system may comprise any embodiment of the ion pumpingsystem (100) described herein. The ion pumping system is configured toseparate the first compartment from the second compartment. In oneembodiment, the second ion-selective membrane (135) is attached to theion transport structure (110) to form a single, continuous barrier thatseparates the first and second compartment. In another embodiment, thesecond ion-selective membrane (135) and the ion transport structure(110) are disconnected from each other, and each one forms a barrierthat separates the first and second compartment. In yet anotherembodiment, the ion pumping system (100) comprised of the interlacedcontact fingers forms a single barrier that separates the first andsecond compartment.

When used in a deionization procedure, each compartment contains thesolution having an initial concentration of ions. When the power source(140) applies an alternating electrical bias to the contacts resultingin a ratchet mechanism, ions from the first compartment are selectivelytransported in one direction across the ion-selective membranes and intothe second compartment, thereby increasing the ion concentration in thesecond compartment and reducing the ion concentration in the firstcompartment. In some embodiments, the desalination system may be used todesalinate or deionize solutions such as salt water, or in chemicalseparations.

EXAMPLE

The following is a non-limiting example of the present invention. It isto be understood that said example is not intended to limit the presentinvention in any way. Equivalents or substitutes are within the scope ofthe present invention.

EPIIR Simulations

The EPIIR theoretical pumping performance was estimated in two stages.First, the potential distribution within the device was calculated usingfinite element simulations (COMSOL Multiphysics). Next, the potentialdistribution obtained in the finite element simulation was used as aninput in an analytic computation to obtain the net ratchet current. ²The finite element simulation domain consisted of a single pore in anAAO wafer and the electrolyte around it. FIG. 2A shows a schematicillustration of the simulation domain. The solid region is assumed to bemade of perfectly insulating alumina and the pore and electrolytecompartments on both sides of the EPIIR are filled with 1 mM KCl aqueoussolution where only the salt ions are considered. The pore diameter is40 nm, the wafer thickness is 50 μm and the thickness of the electrolytecompartments on both sides of the EPIIR is 1.5 μm which is significantlylarger than the Debye length for this salt concentration. The point ofzero charge is assumed to be at a potential of 0 V for all interfaces.At the edges of the simulation domain, the concentrations of salt ionsare assumed to be 1 mM and the potential is set to 0 V thus simulatinginfinitely large compartments on both sides of the EPIIR. FIG. 4A showsthe calculated potential distribution near the EPIIR high voltagecontact at several distances from the pore center, r. The appliedvoltage is 0.4 V. As discussed above, the differences between the Debyelength inside the pore and bulk electrolyte results in an asymmetricelectric potential distribution, which is essential for the ratchetoperation.

In order to insert the two dimensional potential distribution calculatedin the finite elements simulation into the analytic, net particlevelocity calculation, the potential weighted average was calculated.FIG. 4B shows the weighted average near the high voltage contact. Theinset shows a broader view of the potential distribution, and where thepotential is at 0 V outside the region presented. The net particlevelocity, v, is then computed with the weighted average potentialdistribution multiplied by the ratchet signal which is a square wavetemporal function g(t) alternating between 0 and 1 at frequency f andduty cycle.³¹ Since the potential is very close to zero outside a smallregion next to the high voltage contact, only a the region −0.5 μm≤z≤0.1μm was used for the net ion velocity calculation. Once the particlevelocity is obtained, the net ionic current density, J, follows:J=q_(e)nvρ, where q_(e) is the electron charge, n is the concentrationof ions which is taken to be 1 mM, and ρ=0.1 is the EPIIR porosity.

FIG. 4C shows the calculated ionic short-circuit current density for awide range of input signal frequencies and duty cycles. As discussedabove, the voltage switching must be at a period that prevents backflowing ions from leaving the region near the EPIIR high voltagecontact. Thus, if the signal period is too long, back flowing ions willnot be turned upon voltage switching and the net current will be zero.In a similar manner, at duty cycles near zero or one, the systemoperates closer to direct current operation and the net current is againnear zero. The ionic current increases with frequency until the inputsignal switching time is significantly shorter than the period requiredfor forward flowing ions to leave the region next to the EPIIR contact.

The EPIIR response at low frequencies is determined by the time requiredfor back flowing ions to reach the next potential minimum (or maximum).On the other hand, the high frequency response is determined by the timerequired for forward flowing ions to reach a potential minimum (ormaximum) point. Both of these time constants are determined by thediffusivity of the ions. Thus, ions with a different diffusivity have adifferent frequency response and there are specific frequencies in whichions of specific diffusivities will be pumped more efficiently thanothers.

FIG. 5A shows the ionic short-circuit current density as a function ofthe diffusivity of the ions at several input signal frequencies. Thepotential distribution is a saw-tooth distribution as shown in FIG. 5Band the input signal is g(t)=0.5 (1+sin(2πft)). The relative position ofthe contact which forms the point of maximum potential (x_(c)) is at0.75 of the spatial period as shown in FIG. 5B, and the maximumpotential is 0.066 V which is the weighted average potential obtained inthe two-dimensional finite-element simulation. As can be seen in FIG.5A, at every frequency there is a specific diffusivity at which theratchet current is maximal (in absolute values). Thus, by tuning theinput signal frequency, it is possible to choose which ions are pumpedmost efficiently based on their diffusivity. This selectivity enablespumping of specific ions out of mixtures of ions. In some embodiments,the optimal frequencies can range from about 1 GHz-1 MHz, 1 MHz-1 kHz, 1kHz-1 Hz, or 1 Hz-1 mHz. In other embodiments, a maximum frequency maybe about 10 MHz. In some other embodiments, the minimum frequency may beabout 1-10 mHz.

Experimental Validation

EPIIRs were fabricated by electron beam or thermally evaporating 10 nmof titanium as an adhesion layer and then 40 nm of gold (planarequivalent) on each surface of anodized aluminum oxide wafers (InRedoxMaterials Innovation) with various pore diameters. FIG. 6A shows a planview SEM image of one of the EPIIRs fabricated. The ion pumpingproperties of the EPIIRs were tested in an electrochemical cell in whichthe EPIIR served as a membrane separating two electrolyte compartments,each containing an Ag/AgCl wire that was used to probe the potentialbetween the two compartments. FIG. 6B shows a schematic illustration ofthe measurement setup. FIGS. 6C-6E show schematic illustrations anddigital photographs of alternative measurement setups that are alsoamenable to driving overall desalination by pumping at least two typesof ions of oppositely signed charge without performing redox reactions.

The EPIR pumping mechanism was validated by measuring the output ionicopen-circuit voltage, V_(out), for various input signals. The EPIIR has40 nm diameter pores and the aqueous solution is 1 mM KCl. Unless statedotherwise, the input electric signals V_(c)(t) are square waves at afrequency of 100 Hz, with a low voltage level of −0.2 V, and a highvoltage level at 0.2 V. The ratchet input signal was produced with an HP3245A universal source and the voltage between the Ag/AgCl wires wasmeasured with an Agilent 34401A multimeter where both instruments sharedthe same ground. The voltage measurement was conducted with anintegration time of 1.67-3.0 seconds to reduce the output signaloscillations and obtain only a net voltage. The response to every inputsignal was measured for 5 min after which the input was set to 0 V for 5minutes. FIG. 7A shows the recorded ionic open-circuit voltage for dutycycles between 5% and 100% (the duty cycle is the portion of the time inevery period where the voltage is at its high value). The input signalduty cycle is indicated on the plot. Once a ratchet signal commences,the output ionic open-circuit voltage quickly builds up to a leveldetermined by the duty cycle. The ratchet induced voltage reaches itslargest values for duty cycles close to 50%, i.e. a temporally averagedinput voltage of 0 V. For a duty cycle of 100%, which is the response toa voltage step from 0 V to 0.2 V, the voltage signal shows fastcapacitive charging behavior corresponding to polarization of themembrane. However, unlike the response to a duty cycle of 100%, which isa step function at 0.2 V, for a duty cycle of 95%, which is a modulatedinput signal with a temporal average voltage of 0.19 V, the ionicvoltage is negative and with a much slower decay over time. Thisprovides a simple distinction between near-steady-state ratchet-poweredion transport and capacitive charging-discharging behavior.

To estimate the EPIR output ionic open-circuit voltage, the recordedvoltage was averaged over the last 2.5 minutes of every cycle and thedifference between the ratchet ON and ratchet OFF average voltages wascalculated. FIG. 7B shows the EPIIR output ionic open-circuit voltage asa function of the input signal amplitude (peak to peak) for aqueous 1 mMKCl and aqueous 10 mM KCl solutions. The input signal was a square wavewith a voltage offset of 0 V, frequency of 100 Hz and a duty cycle of50%. For the 1 mM KCl solution, a noticeable EPIR output is visible forratchet signals with an amplitude as small as 0.1 V peak to peak. TheEPIIR output is significantly smaller in the 10 mM KCl solution. Withoutwishing to limit the present invention to any theory or mechanism, at ahigher ion concentration, the solution ions better screen the inputsignal and as a result, the center of the pore is less affected by theinput signal and serves as a shunt. FIGS. 7C-7D show the EPIR outputionic open-circuit voltage as a function of the frequency and duty cyclein 1 mM and 10 mM KCl, respectively. Since ratchet systems have nooutput when a constant voltage is applied, the EPIR output ionic voltageis close to 0 V for duty cycles near 0% and 100%. Similarly, at lowfrequencies, the EPIR fully charges and discharges the double layers,which is similar to operation under a constant bias. Thus, the output isnear 0 V at low frequencies as well. As a result, the EPIIR shows asignificant output only when operated with duty cycles near 50% and atinput signal periods that are close to the characteristiccharging-discharging time constant of the EPIIR. When the input signalperiod is significantly shorter than the EPIR charging and dischargingtime constants the output goes to zero again. Since higher salinityresults in lower solution resistivity and shorter time constants, theoptimal frequency for the 10 mM solution is higher than for the 1 mMsolution. The optimal duty cycle is determined by the geometry of thesystem and the input signal properties. For example, in a system wherethe input signal modulation results in a potential alteration as shownin FIG. 3A, the amplitude and the sign of the output ionic current andvoltage will be determined by the input signal duty cycle.

The observation of a net ionic voltage means that the ratcheting processalters the equilibrated condition of the cell to one that can performuseful work. In order to do this, the voltage that is generated must beused. One way to do this is to provide a low-impedanceelectron-transport pathway between the two Ag/AgCl ionic voltage-sensingelectrodes such that net ionic current can flow via intermediateelectronic current generated by redox reactions at the electrodes. Doingthis results in analogous data to that reported for the ionicopen-circuit voltage measurements above, as seen in FIG. 8A, but insteadfor ionic short-circuit current, as seen in FIG. 8B. The cause of theobservation of a net ionic open-circuit voltage and short-circuitcurrent is unclear based on the bulk time-averaged results, but isapparent from individual periods of ionic voltage (FIG. 8C) and ioniccurrent (FIG. 8E), which also clearly illustrate the asymmetry andsignal observed on longer time-averaged data (FIGS. 8D, 8F).

Device Configurations

Stacked EPIIRs

Porous alumina wafers provide a simple substrate that can be used tofabricate EPIIRs. The high mechanical, chemical and thermal robustnessof alumina wafers enable the use of a suite of deposition techniques forthe fabrication of full devices. Furthermore, anodic aluminum oxide(AAO) wafers can be purchased that have nominal pore diameters rangingfrom 10 nm to 250 nm making them compatible with electrolyte solutionshaving a wide range of conductivities. For these reasons, initialstudies focused on EPIIRs based on AAO substrates. The initial simplestdevices to be tested are single EPIIRs fabricated by depositing thinconductive metal layers on both sides of the AAO wafer. FIG. 9A shows aschematic illustration for such a device.

While AAO wafers serve as good EPIIR substrates, other materials, forexample polymers with sub-micron pores, can be used as well. In otherembodiments, similar structures are formed by depositing metal layerswith sub-micron pores on the two sides of an ion selective material suchas Nafion®. Such configurations may have higher ionic conductivity andselectivity and they may be more efficient since the entire ionconducting phase is biased directly and not through fringing fields asin EPIIRs described below. Pores in the metal layers allow the ions tobe easily transported in and out of the device.

The EPIR efficiency can be increased further by using a combination ofan ion selective material such a Nafion as the ion transport layer, anda mixed electronic and ionic conductor such as salts ofpoly(3,4-ethylenedioxythiophene) or a composite material as non-limitingexamples of the conductive layer.⁴⁰ Such a configuration allows theentire surface of the EPIIR to be active instead of just the pores inthe AAO wafer or in the electronically conductive layer described above.

Flashing ratchet devices include periodic structures with asymmetricelectric potential distribution profiles. A similar configuration can beadopted to EPIIRs by stacking several EPIIR structures on top of aporous substrate. The EPIIRs stack can be fabricated by sequentialdeposition of conductive and dielectric layers for example with thermalor electron beam evaporation, sputtering, atomic layer deposition, amongothers. An asymmetric, saw-tooth like electric potential distribution isobtained by alternating the thickness of the dielectric layers betweentwo values. FIG. 9B shows a schematic illustration of a stacked EPIIRand FIG. 9C shows a schematic representation of the electric potentialdistribution within the pore of this a device. The fine control of thethickness of the EPIIRs in this configuration holds a very importantadvantage. Since the absolute amount of ions within the pore depends onthe EPIIR thickness, thinner EPIIRs will have less charge within thepores thus the input potential will not be easily screened by the ionswithin the pore. As a result, thinner EPIIRs will be able to pump ionsin higher concentration solutions. By connecting different metal layersto different power sources, the potential within the pore can becontrolled with more degrees of freedom resulting in a reversibleratchet. In these EPIIRs charge will not disperse in both directionsupon potential switching and as a result, their efficiency can bedramatically higher than that of irreversible ratchets in which thecharged particles disperse forward and backward upon potentialswitching.²⁶ FIG. 9D shows an illustration of an EPIIR that can operateas a reversible ratchet and FIG. 9E shows schematically the spatialelectric potential distribution within its pores.

Semiconducting EPIRs

Undoped and lightly doped semiconducting EPIIRs offer two fundamentaladvantages: They can be operated optically, and formed junctions can beused to fine tune the electric potential distribution within pores. Highaspect ratio pores can be etched with photoelectrochemical etching⁴¹ andwith low temperature inductive coupled plasma etching.⁴² FIG. 10 shows aschematic illustration of semiconducting EPIR made of silicon withjunction formations within it to fine tune the electric potentialdistribution within the pore.

Integrated Devices

Water deionization devices based on EPIIRs and semiconducting EPIIRs canbe constructed with interdigitated devices as shown below. Besides waterdesalination, ion pumps and deionization devices can be used for a rangeof different applications. For example, since different ions areexpected to have a different frequency response, such devices can beused for chemical separations. Devices based on EPIIRs can have improvedfunctionality if integrated together or coupled with ion selectivematerials, for example, by depositing a cation-exchange membrane or ananion-exchange membrane on top of EPIIRs to produce an ion-selectivepump.

Electrodialysis and capacitive deionization systems are hindered bylarge series resistance losses. To reduce these losses, there is a needto design a water deionization system that minimizes the distance thatpairs of oppositely charged ions need to travel, constituting ions thatare actively pumped and those neutralizing counter ions that togetherconstitute a net chemical reaction and even energy storage. In oneembodiment, this distance can be minimized by fabricating interdigitatedEPIIRs. These EPIIRs include two sets of interlaced metal contactfingers, each set connected to different pores. Then, by supplying thetwo sets of contacts with opposite input signals, each set of pores canpump ions in different directions. Cation-exchange membrane andanion-exchange membrane materials are deposited on top of the two setsof contacts making each of the sets selective to either cations oranions. Hence, both types of charged ions can be pumped in aunidirectional manner from one side of the EPIIR to the other.Photolithography can be used to reduce to minimum the distance betweenthe cation and anion pores potentially removing series resistancelosses. FIGS. 11A-11B shows a schematic illustration of theinterdigitated EPIIR. The top view (FIG. 11A) shows the contact fingersand the cation-exchange membrane and the anion-exchange membranedeposited on top of which. FIG. 11B is an illustration of ion transportthrough two adjacent contact fingers. The aforementioned example is butone configuration that can achieve said objective of reducing losses,and it is understood that the present invention is not limited to saidembodiment.

Additional Pumping Processes

Selective Ion Pumping

In carefully designed electronic ratchets, the sign of the ratchetcurrent can be determined by the input signal frequency and is relatedto the position of the charge carriers with respect to potential maximaand minima when the potential is switched. Since this position isdetermined by the diffusivity and input signal frequency, a change indiffusivity may also result in a change in the sign of the current. Thisreciprocity between diffusivity and frequency allowed Skaug andco-workers to demonstrate sorting of nanoparticles according to theirdiffusivity, which is determined by their size and shape, with a tiltingratchet.⁴³ Thus, carefully designed EPIIRs can be made to pump ions ofopposite signs in the same direction, or ions with the same sign inopposite directions, each according to the diffusivity of the ions.

If an aqueous salt solution is composed of anions and cations withdifferent diffusivities, for example, Na⁺ and Cl⁻ with diffusivities of1.33×10⁻⁵ cm²/s and 1.998×10⁻⁵ cm²/s,^(44,45) respectively, there willbe a frequency range over which current reversal will cause cations andanions to travel in the same direction. In this case, as shown in FIG.1G, water can be desalinated in steady state with a single membrane,because both ions are driven in the same direction via a process calledambipolar charge transport. Using a single membrane mimics theopen-circuit condition and in this case has advantages by simplifyingthe design of the EPIIR-based ion pumping system. FIG. 12 shows thecalculated Na⁺ and Cl⁻ short-circuit current densities as a function ofthe input signal frequency. The input signal is g(t)=0.5 (1+sin(2πft))and the relative position of the contact is at 0.99 of the spatialperiod. For frequencies below 950 Hz, both ionic current densities arepositive, i.e. cations and anions flow in opposite directions. However,for frequencies between 950 Hz and 1420 Hz, the Na⁺ current density isnegative while the Cl⁻ current density is positive, resulting inambipolar charge transport where both cations and anions flow in thesame direction. At a frequency of 1162 Hz, the opposing short-circuitionic current densities for each of Cl⁻ and Na⁺ are equal in magnitudeat 2.259 μA/cm², but opposite in sign, meaning there will be no buildupof open-circuit voltage across the membrane, even when a second membraneis not present, which mimics the open-circuit condition. FIG. 13 showsthe ambipolar current density when short-circuit current densities forNa⁺ and Cl⁻ are equal in magnitude and the input frequency thatcorresponds to this condition, for an aqueous NaCl solution as afunction of the relative position of the contact in the spatial period.This ambipolar current density increases dramatically as x, increases,implying that the rate of desalination increases with the asymmetry inthe potential distribution.

An interesting outcome of current reversal based on diffusivity occurswhen one salt is present at a much higher concentration than anothersalt. In this case, a frequency can be chosen to induce chargeseparation of the higher concentration cations and anions, resulting inan ionic open-circuit voltage that opposes their further transportacross the EPIIR. However, in the presence of a salt at a lowerconcentration and with ions of a different diffusivity, these ions canbe transported in the direction of the ionic voltage, resulting inacceleration of their rate of transport and a dramatic enhancement oftheir currents. This can be of great consequence when attempting toseparate ions that are present at a very low concentration within amixture, for example when trying to remove impurity lead ions fromdrinking water.

Water Pumping

Ion transport is also known to induce electro-osmosis, meaning thatEPIIRs not only net transport ions, but also water molecules. There isalso a set of parameters that maximizes the rate of water pumping,meaning that the ratio of ions to water molecules that are pumped acrossthe membrane can be altered by varying the input signal frequency.Because pumping water or ions are effective means to desalinate water,either process can be made to dominate overall pumping through choice ofinput signal frequency and to optimize performance. Electro-osmosis isnot only important for applications in desalination, but also formicrofluidics whereby not driving redox reactions, which typicallyresult in the formation of bubbles as products from water electrolysis,clogging of pores can be mitigated.

As used herein, the term “about” refers to plus or minus 10% of thereferenced number.

Although there has been shown and described the preferred embodiment ofthe present invention, it will be readily apparent to those skilled inthe art that modifications may be made thereto which do not exceed thescope of the appended claims. Reference numbers recited in the belowclaims are solely for ease of examination of this patent application,and are exemplary, and are not intended in any way to limit the scope ofthe claims to the particular features having the corresponding referencenumbers in the drawings. In some embodiments, the figures presented inthis patent application are drawn to scale, including the angles, ratiosof dimensions, etc. In some embodiments, the figures are representativeonly and the claims are not limited by the dimensions of the figures. Insome embodiments, descriptions of the inventions described herein usingthe phrase “comprising” includes embodiments that could be described as“consisting essentially of” or “consisting of”, and as such the writtendescription requirement for claiming one or more embodiments of thepresent invention using the phrase “consisting essentially of” or“consisting of” is met.

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What is claimed is:
 1. An ion transport structure (110) comprising anion-permeable layer (112) coupled to at least two contacts (122),wherein the ion transport structure (110) is configured to transportions across the ion-permeable layer (112) when an input signal frequencyis applied to change electric fields within the ion transport structure,resulting in a ratchet-driven ion pump.
 2. The ion transport structure(110) of claim 1, wherein the ion transport structure is configured tocontinuously transport ions using alternating electronic polarization,wherein the ion transport structure (110) pumps ions with minimizedresistance and without using electrochemical reactions or mechanicalforces.
 3. The ion transport structure (110) of claim 1, wherein theion-permeable layer comprises a dielectric material, a semiconductor, apolymer, or an ion-selective material.
 4. The ion transport structure(110) of claim 1, wherein the contact (122) is a layer or wire comprisedof an electrically conductive material.
 5. The ion transport structure(110) of claim 1, wherein a plurality of channels (115) is disposedthrough the ion transport structure (110).
 6. A method of selectivelymoving one or more types of ions in a solution, said method comprising:a. providing the ion transport structure (110) of claim 1: b. placingthe ion transport structure (110) in the solution; and c. choosing aninput signal frequency based on a diffusivity of the one or more typesof ions in the solution that modulates a spatially asymmetric electricpotential distribution to change electric fields within the iontransport structure to optimally transport the one or more types of ionsacross the ion-permeable layer (112) in either direction.
 7. The methodof claim 6, wherein the one or more types of ions in the solution are ofa same charge type and are transported in opposite directions, resultingin steady state ion pumping.
 8. The method of claim 6, wherein the oneor more types of ions in the solution are of opposite charge types andare transported in the same direction, resulting in steady state ionpumping.
 9. The method of claim 6, wherein the transport of ions in thesolution causes a net flux of water, thereby pumping water across theion transport structure.
 10. An ion transport structure (110) comprisinga plurality of ion-permeable layers (112) and a plurality of contacts(122) forming a stack in which the ion-permeable layers (112) alternatewith the plurality of contacts (122), wherein the ion transportstructure (110) is configured to transport ions through the stack whenan input signal frequency is applied to change electric fields withinthe ion transport structure, resulting in a ratchet-driven ion pump. 11.The ion transport structure (110) of claim 10, wherein a plurality ofchannels (115) is disposed through the stack of alternating layers. 12.An ion pumping system (100) comprising: a. an ion transport structure(110) comprising an ion-permeable layer (112) and at least two contacts(122) coupled to the ion-permeable layer (112); b. a first ion-selectivemembrane (130) operatively coupled to the ion transport structure (110);and c. a second ion-selective membrane (135) operatively coupled to theion transport structure (110); wherein the first ion-selective membrane(130) and the second ion-selective membrane (135) are each selective forions having a specific charge, wherein the ions are transported acrossthe ion transport structure (110) and the second ion-selective membrane(135) when an input signal frequency is applied to change electricfields within the ion transport structure, resulting in a ratchet-drivenion pump.
 13. The ion pumping system (100) of claim 12, wherein the iontransport structure is configured to continuously transport ions usingalternating electronic polarization, wherein the ion transport structure(110) pumps ions with minimized resistance and without usingelectrochemical reactions or mechanical forces.
 14. The system (100) ofclaim 12, wherein the first ion-selective membrane is disposed on theion transport structure and the second ion-selective membrane (135) isattached to the ion transport structure (110) such that the secondion-selective membrane (135) and ion transport structure (110) are sideby side.
 15. The system (100) of claim 12, wherein a plurality ofchannels (115) is disposed through the ion transport structure (110).16. The system (100) of claim 15, wherein the at least two contacts(122) comprise two sets of interlaced contact fingers, each setconnected to different channels, wherein a first set of stripscomprising the first ion-selective membrane (130) is disposed on one setof contact fingers and a second set of strips comprising the secondion-selective membrane (135) is disposed on the other set of contactfingers such that the strips of the first ion-selective membrane (130)alternate with the strips of the second ion-selective membrane (135),wherein each set of contact fingers has a corresponding set ofinterlaced contact fingers disposed on the second surface (116) of theion-permeable substrate and connected to the same channels, therebyforming a paired set, wherein each paired set is connected to its ownseparate power source.
 17. A deionization system for moving ions in asolution from a first compartment to a second compartment, comprisingthe ion pumping system (100) of claim 12, wherein the ion pumping systemseparates the first and second compartment, wherein each compartmentcontains the solution having an initial concentration of ions, whereinwhen an input signal frequency is applied to change electric fieldswithin the ion transport structure, resulting in a ratchet mechanism,ions from the first compartment are selectively transported in onedirection across the ion pumping system, thereby increasing the ionconcentration in the second compartment and reducing the ionconcentration in the first compartment.
 18. The deionization system ofclaim 17, wherein the first ion selective membrane (130) is disposed onthe ion transport structure (110) and the second ion-selective membrane(135) is attached to the ion transport structure (110) to form a single,continuous barrier that separates the first and second compartment. 19.The deionization system of claim 17, wherein the first ion selectivemembrane (130) is disposed on the ion transport structure (110), whereinthe second ion-selective membrane (135) and the ion transport structure(110) are disconnected, and each one forms a barrier that separates thefirst and second compartment.